It isn't particularly straightforward to add a pedal box to an
Pedal-Electric Duo-Arts [PEDA] have a completely different form of
expression box, with three sets of large valves inside. One is the
themeing for Duo-Art (switches the stack from accompaniment to theme
suction); the second is the themeing for foot playing (switches the
stack from accompaniment to unregulated supply suction); the third
set isolates the theme side of the Duo-Art box when in foot-pump mode.
Electric-only Duo-Arts only have the first set of valves.
The pedal-electric system also requires the standard Themodist manual
controls for accompaniment. There are also additional controls
surrounding this, such as two sets of theme primary valves and a
crossover switch to operate the right set according to playing mode.
There is also a switch that automatically disables the theme side of
the box when foot-playing, even if Duo-Art mode is selected.
Some British Weber Duo-Art grands were equipped with the 6-valve
expression box even if sold in electric-only form. Other than these,
conversion requires major changes to the expression system --
effectively, an entirely new expression box. Not a particularly
sensible thing to do!
The largest British Weber, the 6-foot model 12 grand, in Duo-Art form
seems to be in plentiful supply at the moment. These pianos cost very
nearly as much as a Steinway when new (850 guineas vs. 1050 guineas in
1924), but nowadays are far cheaper even though they are superb
instruments. I'd buy one if I didn't have one already!